1. Field of the Invention
The present invention relates to radiation curable fluids, particularly to radiation curable inkjet inks for industrial inkjet printing methods.
2. Description of the Related Art
In industrial ink jet systems, there is a constant demand for increased printing speeds in combination with high image quality. The new print heads, designed for increasing printing speed, only operate with very low viscous inkjet inks. Radiation curable inkjet inks are particularly preferred because high quality images can be printed on non-absorbing ink-receivers. Suitable monomers to obtain such very low viscous ink jet inks have been described, for example, in EP 997508 A (AGFA) that discloses radiation curable monomers containing vinylether and acrylate functions.
Printing systems, such as offset and flexography, are being increasingly replaced for packaging applications by industrial inkjet printing systems due to their flexibility in use, e.g. variable data printing, and due to their enhanced reliability, allowing their incorporation into production lines, e.g. for printing on packaging. The use of low viscous vinylether acrylate monomers has revealed some drawbacks for reliability, especially regarding yellowing and constant ink viscosity.
Yellowing is a well-known degradation process in the polymerization of monomers, which may be initiated or accelerated by numerous factors including ultraviolet light (UV), visible light, ozone and other extraneous pollutants, intrinsic manufacturing impurities, oxygen, and heat.
One approach to reduce this deterioration is to add stabilizers such as antioxidants and UV stabilisers during manufacturing of the radiation curable fluids. However, the addition of UV stabilizers tends to reduce the curing speed of a UV curable fluid. US 2008299489 A (HP) discloses an ultraviolet curable coating fluid including a polymerizable olefin monomer or monomer blend that undergoes self-photoinitiating polymerization when exposed to a predetermined ultraviolet wavelength range, and a predetermined amount of an ultraviolet absorbing image stabilizer that has minimal absorption in the predetermined ultraviolet wavelength range.
US 2011183081 A (SEIKO EPSON) discloses a photocurable ink composition including a polymerizable compound; an acylphosphine oxide-based photopolymerization initiator; and up to 2 wt % of a stabilizer selected from a specific hindered amine-based compound and a hydroxyphenyltriazine-based compound.
Another approach is disclosed by US 2011196058 A XEROX) which uses optical brighteners to mask yellowing thus improving the appearance of an image.
However, most approaches target the photoinitiating system, since often discoloration is due to decomposition of the photoinitiating system.
The photoyellowing can be reduced by using specific photoinitiators that do not exhibit photoyellowing. EP 2108683 A (XEROX) discloses a substantially colourless radiation overcoat composition including a gellant, a monomer and a substantially non-yellowing photoinitiator selected from the group consisting of benzophenones, benzoin ethers, benzil ketals, α-hydroxyalkylphenones, and acylphosphine photoinitiators.
WO 2011/069943 (AGFA) discloses a UV LED curable composition including an aliphatic tertiary amine or dialkyl aniline co-initiator and a carbazole photoinitiator, which can be advantageously used to prevent unstable yellowing behaviour in an image upon storage.
The use of smaller amounts of photoinitiator usually also reduces the photoyellowing. Thioxanthone photoinitiators are known to form several degradation products with a limited stability. As a result, the original yellowing shifts upon storage of inkjet prints, this unstable yellowing behaviour makes control of the image tone in the final image difficult. EP 2053102 A (AGFA) discloses a radiation curable composition including at least 25 wt % of a vinylether acrylate and at least 15 wt % of a polymerizable compound comprising at least three acrylate groups; and a photoinitiator comprising a tertiary amine group and 0 to 3 wt % of isopropylthioxanthone.
US 2007066700 A (SUN CHEMICAL) discloses that the piperazine sensitizers of the invention cause yellowing and may only be used successfully in varnishes if this is not of importance.
EP 2399965 A1 (AGFA) discloses a free radical radiation curable inkjet ink containing a photoinitiator and polymerizable compounds including at least 45 wt % of a mixture of monomers consisting of: a) 10 to 35 wt % of one or more cyclic monofunctional acrylates wherein the homopolymer thereof has a Tg larger than 20° C.; b) 10 to 30 wt % of a N-vinyl lactam and/or a vinylether acrylate; c) 10 to 30 wt % of an ethoxylated and/or propoxylated polyacrylate having a molecular weight of at least 450; wherein all wt % are based on the total weight of the inkjet ink. An ink is disclosed containing C.I Pigment Blue 15:4, the dispersant Disperbyk™ 162 and VEEA.
EP 2053103 A1 (AGFA) discloses a A free radical curable liquid for inkjet printing of food packaging materials comprising no initiator or otherwise one or more initiators selected from the group consisting of non-polymeric di- or multifunctional initiators, oligomeric initiators, polymeric initiators and polymerizable initiators; wherein the polymerizable composition of said liquid consists essentially of: a) 25-100 wt % of one or more polymerizable compounds A having at least one acrylate group G1 and at least one second ethylenically unsaturated polymerizable functional group G2 different from the group G1; b) 0-55 wt % of one or more polymerizable compounds B selected from the group consisting of monofunctional acrylates and difunctional acrylates; and c) 0-55 wt % of one or more polymerizable compounds C selected from the group consisting of trifunctional acrylates, tetrafunctional acrylates, pentafunctional acrylates and hexafunctional acrylates, with the proviso that if the weight percentage of compounds B >24 wt %, then the weight percentage of compounds C>1 wt %; and wherein all weight percentages of A, B and C are based upon the total weight of the polymerizable composition; with the proviso that at least one polymerizable compound B or C is present in the polymerizable composition if the free radical curable liquid contains no initiator; wherein the polymerizable compound A has a copolymerization ratio of with r 1 and r 2 representing the copolymerization parameters of methyl-G1 respectively methyl-G2 determined according to the method of Kelen-Tudos.
It is also possible to eliminate the photoinitiating system by using electron beam curing instead of UV curing. However, in the case of vinylether acrylate monomers a profound yellowing could be observed, especially when compounds having sulfonic acid groups were present. This can be especially well observed for cyan and white radiation curable inks, which after printing and curing result in a greenish cyan respectively a yellowish white colour. Also an increase in ink viscosity was generally seen.
Therefore, there is a need to improve radiation curable fluids and inkjet inks including vinylether acrylate monomers that exhibit no or only minor photoyellowing and constant ink viscosity, resulting in a reliable industrial ink jet system.